Embodiments of the subject invention pertain to the field of shielding of RF coil arrays in magnetic resonance (MR) imaging at high frequencies. Currently, shielded birdcage coils and transverse electromagnetic (TEM) coils (or transmission line resonators) are the most widely used volume coils for MR imaging using static magnetic fields of 4 T and above. A birdcage coil typically has a plurality of conductor elements (legs), which run parallel to each other from one end of the volume coil to the other end. At each end of the birdcage coil, the legs are electrically connected to an end ring, which is a closed ring in electrical contact with each leg conductor. In contrast, a TEM coil typically has a plurality of conductor elements (legs), which run parallel to each other from one end of the volume coil to the other end and are not electrically connected at each end of the TEM coil. In this way, each leg of a TEM coil can be driven independently of the others.
Additionally, surface coils or transmission line resonators are often used in a collection of weakly coupled elements that can be driven simultaneously to provide volume-like effects, with the ability to adjust the magnetic field by controlling the amplitude and phase of the drive for each element. The principle advantage of the birdcage coil is that it produces a relatively uniform field (in the unloaded state) and maintains a strong separation between modes because of the relatively strong coupling between adjacent meshes. This makes the field of a particular mode less sensitive to perturbation by errors in symmetry or loading properties of the sample. By contrast, the TEM coil has weaker coupling and, therefore, several modes of fairly low Q can interact, thus requiring multiple locations for drives in order to produce the desired field. For a specific sample the TEM coil can be adjusted to compensate for asymmetries in construction and the effects of the sample loading.
A convenient way to model bulk interactions of samples and volume coils at high frequency is to consider the sample and shield of the coil or bore through the formalism of cavity resonators and waveguides. Introduction of the sample into the volume of the volume coil can change the resonate wavelength and create standing waves and modes. This effect kicks in for a certain frequency to sample size ratio (e.g., when sample is on the order of a wavelength). Application of this analysis demonstrates that the TEM current elements couple differently to various cavity resonant modes than a birdcage coil, because of the end-ring currents of the birdcage.